The invention relates to a low-pressure mercury discharge lamp with a very good colour rendering and with a colour point (x.sub.L,y.sub.L) on or close to the Planckian locus, provided with a gastight, radiation-transmitting envelope which contains mercury and rare gas, and provided with a luminescent layer comprising
a first luminescent material activated by bivalent europium and having an emission band whose maximum lies between 470 nm and 500 nm and whose half-value width is at most 90 nm, PA1 a second luminescent material activated by bivalent manganese and having at least an emission band mainly in the red region of the visible spectrum. PA1 a third luminescent material with a band emission whose maximum lies between 430 nm and 490 nm, PA1 a fourth luminescent material with emission mainly between 520 nm and 565 nm, and PA1 a fifth luminescent material with emission mainly between 590 nm and 630 nm.
The expression "a very good colour rendering" in the present description and claims is understood to mean that the average colour rendering index R(a,8) (average of the colour rendering indices of eight test colours as defined by the International Commission on Illumination: CIE Publication No. 13.2 (TC-3.2), 1974) has a value of at least 90.
The colour of visible radiation is characterized by the colour coordinates (x,y) which determine the colour point in the chromaticity diagram (see CIE Publication No. 15 (E-1.3.1), 1971). A low-pressure mercury discharge lamp, also referred to as lamp hereinafter, for general lighting purposes should emit light which can be designated as "white". White radiation is found in the colour triangle for colour points situated on the Planckian locus. This curve, also called the blackbody line and referred to hereinafter as curve P, contains the colour points of the radiation emitted by a perfectly black body at different temperatures (the so-called colour temperature). A certain colour temperature is assigned, not only to a given point on curve P, but also to radiation having colour coordinates situated on a line intersecting the curve P in said point (see the cited CIE Publication No. 15). If this radiation has a colour point close to the curve P, this radiation is also regarded as white light having this given colour temperature. In the present description and claims, "a colour point close to the curve P" is understood to mean that the distance from the colour point to the point on the curve P having the same colour temperature is at most 20 MPCD. MPCD ("minimum perceptible colour difference") is the unit of colour difference, see the publication by J. J. Rennilson in Optical Spectra, October 1980, p. 63. Apart from requirements to be imposed on the luminescent layer in view of properties regarding the colour temperature of the light radiated by the lamp and the colour rendering, it is also required that the luminescent layer comprises substances which upon excitation by the ultraviolet radiation generated in a low-pressure mercury discharge yield a high luminous flux and maintain this high luminous flux throughout lamp life.
Low-pressure mercury discharge lamps radiating white light and having a very good colour rendering are known from European Patents EP-A-0114441 and EP-A-0124175. It is necessary in these lamps for the luminescent layer to comprise in addition a calcium halophosphate activated by antimony or by antimony and manganese in order to obtain the desired lamp properties. It is found in practice that the requirements as to the colour of the light radiated by the lamp and the colour rendering can only be realised in the known lamp in combination with a comparatively low luminous efficacy of the low-pressure mercury discharge lamp. This is particularly the case when there is a comparatively high wall load in the lamp because in highly loaded lamps the detrimental influence of the very short-wave ultraviolet radiation (185 nm) generated in the mercury discharge plays a major part. A comparatively strong degradation of the luminescent layer may also occur during life of the known lamp.